I. Field of the Invention
The present invention relates to multi-phase fluid flow measurement, and more particularly, to a system and method for three-phase fluid flow rate measurement.
II. Description of the Related Art
Generally, oil, gas and water simultaneously flow from a producing well. A production mixture of oil, gas and water is often referred to as a three-phase flow of oil, gas and water. The term "phase", as used herein, refers to a type of fluid that may exist in contact with other fluids. A mixture of oil and water, for example, includes a discrete oil phase and a discrete water phase. Similarly, a mixture of oil, gas and water includes a discrete gas phase and a discrete liquid phase, with the liquid phase including an oil phase and a water phase.
It is often necessary to measure the oil, gas and water flow rates, Q.sub.o, Q.sub.g and Q.sub.w, respectively, of a three-phase fluid flow. The ratios or fractions of oil, gas and water of a three-phase fluid flow are referred to in the industry as oil, gas, and water cuts, respectively.
Consider a three-phase fluid flow in a pipeline composed of oil, water and gas phases.
The oil, water and gas flow rates in the pipeline are at the respective rates of Q.sub.o, Q.sub.w, and Q.sub.g. Suppose, at any point along the axial length of the pipeline, the volumetric fractions of oil, water and gas are X.sub.o, X.sub.w, and X.sub.g, respectively. The volumetric fractions are represented by the following equations. EQU X.sub.o +X.sub.w +X.sub.g =1 (1) EQU Water Cut W.sub.c =X.sub.w /(X.sub.o +X.sub.w) (2) EQU Oil Cut O.sub.c =1-W.sub.c (3) EQU Gas Cut G.sub.c =X.sub.g /(X.sub.o +X.sub.w) (4) EQU Gas Volume Fraction GVF=Q.sub.g /(Q.sub.g +Q.sub.o +Q.sub.w)(5)
The flow rates and volumetric fractions are related by the following equations. EQU X.sub.o =Q.sub.o /(Q.sub.w +Q.sub.o) (6) EQU X.sub.w =Q.sub.w /(Q.sub.w +Q.sub.o) (7) EQU X.sub.g =Q.sub.g /(Q.sub.w +Q.sub.o) (8)
Multi-phase fluid flow measurement systems, also known as multi-phase fluid flow meters (MPMs), are used to measure Q.sub.o, Q.sub.g and Q.sub.w of a three-phase fluid flow. Three types of MPMs differing in degree of fluid separation are generally available: three-phase separation MPM; two-phase separation MPM; and no-separation MPM. Conventional threephase and two-phase separation MPMs often suffer from the disadvantages of large size and poor accuracy. They often require large separators to substantially separate the gas component from the liquid component of a multi-phase fluid flow. A separator completely separates a multi-phase fluid into its respective phases such water, oil and gas, or simply liquids and gas.
Industry terminology refers to a "two-phase" separator as one that is used to separate a gas phase from a liquid phase including oil and water. The output streams of a two-phase separator are a liquid stream in a liquid leg and a gas stream in a gas leg. A three-phase separator is used to separate the gas phase from the liquid phase and also separates the liquid phase into oil and water phases. The output streams of a three-phase separator are an oil stream in an oil leg, a water stream in a water leg and a gas stream in a gas leg. Each stream may be represented by a fluid rate and a fluid fraction.
As compared to two-phase separators, three phase separators require additional valves and other assemblies. Also, three-phase separators typically have larger volumes to permit longer residence times of produced materials for gravity separation of the production materials into their respective oil, gas and water components.
As noted before, conventional MPMs require large separators to substantially separate the liquid and gas components of the multi-phase fluid flow. Without a substantial separation of the liquid and gas components, conventional systems do not accurately measure the water, oil and gas cuts of the multi-phase fluid flow. A large separator, however, adds size and weight to the overall system, thereby making it less desirable in some locations.
Additionally, some conventional no-separation MPMs measure the various ratios accurately only if the gas cut is within a preferred range. One system, for example, operates reliably if the gas cut is between 60%-70%. Others operate satisfactorily provided that the gas cut is between 70-85%. Thus, most conventional systems are either too large or are not very reliable over a wide range of gas cut.
For these reasons, there is a need for a MPM that is accurate and reliable over a wide range of gas cut. Furthermore, there is a need for a MPM that is smaller in size compared to a conventional system.